I don't know that you would want to say something negative about your old boss in your letter.. that might make you look bad.

I think Karen's first line is great though - let them know you are interested! If you are worried your old boss said somehting negative, just say something else about what is positive about you and really try to sell yourself. If they specifically say that you got a poor recommendation, then say something but you might sound kind of paranoid and defensive if you are the one to bring it up